Two forms of retinal disease include Stargardt disease, which afflicts young adults, and age related macular degeneration (AMD), which afflicts adults in midlife and later. Both forms are characterized by the progressive degeneration of cone photoreceptors located in the foveal region of the macula, which degeneration leads to loss of high acuity vision in the central visual field. The disease has been associated with the accumulation of toxic biochemicals, including lipofuscin, inside retinal pigment epithelium (RPE) cells and extracellular drusen where the RPE cells are in contact with Bruch's membrane. The accumulation of these retinotoxic mixtures is one of the most important known risk factors in the etiology of AMD.
AMD begins as a “dry form” without vascular complications. Currently there are no known treatments for dry form AMD. One patient in ten progresses to a late-stage form of the disease known as “wet form” AMD which is characterized by choroidal neovascularization that invades the macula and disrupts retinal and RPE tissue. Most current wet form AMD treatments suppress vascular growth or inflammatory processes.
During the normal visual cycle (summarized in FIG. 1), most trans-RAL is sequestered by opsin proteins in photoreceptor outer segment disc membranes. This sequestering mechanism protects the trans-RAL group from reacting with phosphatidylethanolamine (PE) before trans-RAL dehydrogenase (RDH) converts trans-RAL to the alcohol trans-retinol. Some trans-RAL molecules escape sequestering, however, and react with phosphatidylethanolamine to form first N-retinylidene-phosphatidylethanolamine (APE) and then N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE) in the discs of photoreceptor outer segments. Both A2PE and trans-RAL that has escaped sequestering are transported out of photoreceptor disc membranes by an ATP-binding cassette transporter called Rim protein (RmP) or ABCA4 (formerly ABCR). Following this transportation, trans-RAL is reduced to trans-retinol by RDH and crosses the outer-segment (OS) plasma membrane into the extracellular space where it is taken up by cells of the retinal pigment epithelium (RPE).
A2PE is taken up by RPE cell lysosomes when RPE cells ingest photoreceptor outer segments that are shed routinely. Once inside the lysosomes, A2PE is converted irreversibly to A2E, which causes lysosomal failure. Lysosomal failure poisons the RPE cells and compromises their ability to provide biochemical support to retinal photoreceptors, leading to the progressive degeneration of both cell types.
Multiple factors affect the rate of A2E accumulation, both genetic and environmental. For example, a hereditary mutation in both copies of the ABCA4 transporter gene increases the accumulation of A2E and leads to Stargardt disease in children and young adults. A later onset form of Stargardt disease is associated with ABCA4 mutations that are more benign. Stargardt disease is thought by many to be an early onset form of AMD, where the normal age-related accumulation of A2E is accelerated by the ABCA4 mutation to a sufficient extent that the disease is triggered decades before AMD normally appears.
With respect to environmental factors, it is well established in animal models that the rate of A2E formation varies with light exposure. It has been shown that a fatty acid (phosphatidylglycerol) can protect RPE cells from A2E induced cell death, and that other dietary factors can influence disease progression, including zinc (which affects retinol oxidoreductase activity).
There is a need for effective treatments of dry form AMD and Stargardt disease which arrest disease progression and preserve or restore vision.